1. Field of the Invention
This invention relates to amplification of signals and, more particularly, to controlling amplifier gain in the amplification of high frequency signals.
2. Description of Related Art
Amplifiers are an important component in almost all electronic circuits. Amplifiers provide a means by which the amplitude of an input signal can be increased while maintaining the relative time-amplitude relationship of the input signal at each point in time. Amplifiers can be constructed to use feedback, which comprises taking a portion of the output signal and returning it to the amplifier input to form the signal that drives the amplifier. Generally, there are two types of feedback. One type of feedback is called positive feedback, and the other type of feedback is called negative feedback.
Positive feedback means that all or part of the amplifier output signal is added to the input of the amplifier to increase the input and thus increase the amount of gain the amplifier provides with respect to the gain that otherwise would be present in the absence of the positive feedback. Positive feedback is commonly considered to be undesirable, because positive feedback tends to destabilize an amplifier (i.e., tends to cause the amplifier output to oscillate). The exception to this general rule is amplifiers that are intentionally destabilized to generate an oscillating output signal at a desired frequency.
Amplifiers become unstable because very small amplitude inputs (such as random noise that typically occurs upon initial application of power to the amplifier) will be amplified at the output and will cause the input signal to increase. Because the input signal is then amplified by the amplifier, the amplitude at the output increases. The output is then fed back to the input, further increasing the input. It should be apparent that this cycle will continue until the amplifier is saturated and any change in the input will be of no effect at the output.
The second type of feedback mentioned above is called negative feedback. Negative feedback means that all or part of the output signal from the amplifier is subtracted at the input of the amplifier to reduce the input and thus lower the gain of the amplifier with respect to the gain that otherwise would be present in the absence of the negative feedback. Negative feedback may have a number of positive effects. First, negative feedback can stabilize an amplifier by reducing the gain of the amplifier and mitigating oscillation of the amplifier output. Stabilizing an amplifier means making the gain less dependent on the specific device parameters and reducing the chance that spurious oscillations in the amplifier will occur. Second, negative feedback can permit amplifiers to achieve more linear operation. Third, negative feedback can permit an amplifier to have a broader bandwidth of acceptable operation. Fourth, negative feedback can be used to lower the input and output impedance of the amplifier. Fifth, negative feedback can be used to reduce the noise in the amplifier output and reduce thermal effects.
Two particular types of negative feedback can be distinguished on the basis of the action of the feedback on the amplifier gain. These two types of feedback are called series feedback and shunt feedback. In general, a shunt feedback amplifier configuration is an amplifier with a feedback network connected across the amplifier input and output terminals. Shunt feedback amplifiers are widely used due to the many desirable properties associated with them. For example, shunt feedback amplifiers have well-controlled input and output impedances, improved stability, and improved amplitude slope and flatness.
One problem that occurs in the practical application of shunt feedback amplifiers is that at relatively high frequencies an excessive phase shift in the output signal can be introduced by the feedback path. More particularly, for negative feedback, it is desirable to provide a feedback signal with phase opposite that of the input signal, to limit the gain applied by the amplifier. If the length of the feedback path is too large for the frequencies involved, then the feedback path will introduce a phase shift that will cause undesirable, premature roll-off of the amplifier output. A feedback path of proper length will provide precisely optimally phased feedback that will increase the roll-off frequency.
It should be understood that the length of the feedback path is relative to the wavelength of the signals to be amplified. That is, the higher the frequencies at which the amplifier is required to operate, the shorter the feedback path must be. With some devices, a relatively long shunt feedback path may be necessary to physically couple the amplifier input to the amplifier output. The longer feedback paths can result in a reduction in the bandwidth over which the amplifier operates with a relatively flat frequency response. This is especially the case for high frequency (20-30 GHz) applications.
For example, a hybrid microwave integrated circuit (HMIC) is a circuit where an interconnect pattern and distributed circuit components are printed onto a substrate, and active and lumped circuit components are individually attached to the interconnect pattern with soldering and wire bonding techniques. The HMIC can have a hole cut through the substrate called a transistor cavity or chip hole, into which a single-chip transistor can be located and wire-bonded to the substrate. The length of a feedback path for the single-chip transistor will be limited by the dimensions of the chip hole and of the HMIC. For amplifier applications in the 20-30 GHz band, it is often impractical to provide a feedback path that is physically short enough to provide an optimally phased shunt feedback signal.
From the discussion above, it should be apparent that there is a need for a shunt feedback amplifier with improved performance in high frequency applications. The present invention fulfills this need.